The present invention relates to an electric motor control system which drives an electric motor based on the prediction of a change in the DC voltage applied to a power converter; and also a series hybrid vehicle, an electric motor control apparatus, and an electric motor control method, based on the electric motor control system.
As disclosed in JP-A-2000-166009, there is known a series hybrid vehicle which includes an engine, an electric motor coupled to the engine and having a rectifier, a battery for being charged with an DC output of the motor, an inverter for converting a DC power charged by the battery into an AC power, an electric motor driven by a rectangular wave voltage generated by the inverter, and a drive mechanism for transmitting an output of the electric motor to wheels.
Since driving or tractive forces of mining or construction vehicles are extremely larger as compared with those of passenger cars, the mining or construction vehicle is generally an engine-driven vehicle which drives the wheels by engine power from the engine through a transmission of the vehicle. Such an engine-driven vehicle, however, has defects that braking operations cause considerable wear of brake pads and that a mechanical component such as the transmission has a short periodical exchange interval. From a viewpoint of a demand of reducing a maintenance cost and also from another viewpoint of such a limitation that a battery commensurate with an amount of necessary charged energy is too expensive, it is considered to utilize such a series hybrid vehicle requiring no battery mounted thereon.
In the engine-driven vehicle, in order to change the driving torque of wheels, it is necessary to change the output torque of an engine as a power source. However, this method is defective in that individual wheel torque adjustment cannot be made because the torques of all the wheels are changed at the same time, and that it is hard to make the torque adjustment immediately due to the influences of the response waste time and response time of the engine itself. The series hybrid vehicle not mounted with a battery utilizes the machine inertial energy of an engine generator and the electric energy of a DC circuit. A series hybrid vehicle mounted with a battery, however, can change the individual torques of wheels from the torque of an electric motor at high speed independently of each other by utilizing an electric energy charged in the battery. This can be attained, because the torque control of the engine and the torque control of the electric motor are independent of each other and also the motor torque control is much shorter in response time than the engine torque control.
A means for controlling the electric motor, further, is controlled to perform dq vector control for control of the current of the electric motor by dividing the electric motor current into an excitation magnetic flux component (d-axis component) and a torque current component (q-axis component) perpendicular to the excitation magnetic flux component, and a torque command is converted to a q-axis current command so as to make the electric motor current converged into the q-axis current command. In other words, the speed of the series hybrid vehicle is controlled by controlling the driving force (torque) of the motor on the basis of a motor torque command (torque current command) based on the amount of accelerator pedal depression (throttle opening) or the amount of brake depression.
When the q-axis component of an AC voltage (nearly synonymous with the amplitude of the AC voltage) is applied to the electric motor, the excitation magnetic flux component (d-axis component) is established with a time lag of first order as a response time constant (e.g., 1.3 seconds) inherent to the electric motor. The torque current command (q-axis component) is established with a response time constant (e.g., 0.1 seconds) to the motor control means. With respect to the motor torque proportional to a product of the magnetic flux component (d-axis component) and the torque current (q-axis component), in order to satisfy a demand of speeding up the control response of the motor torque, it is general to avoid or minimize a change in the excitation magnetic flux component (d-axis component) having a long response time constant. More specifically, the excitation magnetic flux component (d-axis component) in a low speed region is kept to have a constant value, and the excitation magnetic flux component in a high speed region (d-axis component) is gradually weakened from the viewpoint of a limitation that an applied voltage is required to have a constant voltage characteristic (weak excitation).